Here's my reasoning:
A 25 man (Correct me if im wrong) are generally sending 3-4 people down per totem. That leaves 21-22 people outside that the shadowy minions could potentially target with their Spiritual Grasp. Lets go even further and say theres two groups down on 25 man, and subtract 6-8 people. That still leaves 17-19 potential targets.
Whereas a 10 man can only have 7-8 people on the outside when they send people down to the spirit world. So if you have the same spawn rate, i'm sure you see what the issue is.
So the inherent issue (All based on the confirmation that the spawn rate is the same, and that the only difference is the HP) is that the 10 mans have a higher chance at getting gibbed since there's less people to target with the Spiritual Grasp so that one person can take several spiritual grasp hits all within a second.
So basically i'd just like to know if this is indeed the case, or if 25 mans have a faster spawn rate.